1. Field of the Invention
The invention relates generally to mass measuring and more particularly to an automatic zero load adjustment mechanism for a weighing system.
2. Brief Description of the Prior Art
A typical zero load adjust mechanism for spring scales employed an auxiliary biasing spring and has been disclosed in U.S. Pat. No. 3,806,517. One end of the auxiliary spring was adjustably anchored to the scale frame, while the other end was connected to the scale tare structure. The zero load tare position was adjusted through variation of the preload force applied to the tare by the auxiliary spring. Adjustment was accomplished with a threaded shaft and nut at the fixed anchor.
Utilization of auxiliary biasing springs for no load adjustment of scales has presented several disadvantages which have not only detracted from the accuracy of weight determinations but have resulted in increased costs. Initially, zero adjust mechanisms added costs due to the cost of the biasing spring, its associated mountings and the added assembly costs. It should be noted that the biasing spring itself was required to be thermally matched with the main scale spring.
Naturally, the adjust mechanism introduced error due to the addition of a further moving part in the scale. Such error could be attributed to numerous factors, including friction loss at the biasing spring mountings, spring hysteresis, etc. Further, as the compensating force applied by the biasing spring increased, error increased. Another disadvantage encountered with the use of zero biasing springs was that these relatively small springs were susceptible to vibration which was especially undesirable in high speed weighing environments.
In U.S. Pat. No. 3,917,012 an automated zero load adjust mechanism has been disclosed in conjunction with a high speed automated mail processing system for sorting, weighing and imprinting postage on large quantities of mixed mail. The system employed in optical readout apparatus for detecting the deflection of a weighing scale tare when supporting a piece of mail. This prior apparatus included a bank of multiple photodetectors at the end of a light path. A shutter carried by the tare was operable to gradually block the light path to successive photodetectors of the bank. A signal indicative of the number of photodetectors remaining in the light path, hence tare deflection, was produced and utilized to set the postage to be imprinted on the piece of mail.
The system employed an automatic zero adjust mechanism which varied a preload force applied to the scale tare by an auxiliary biasing spring. The first photodetector of the bank sensed deviation of the scale tare from zero load position and actuated a motor and worm drive to adjustably vary the position of one end of the biasing spring, the other end of which was linked to the tare.
This automatic system encountered problems relating to motor shut-off characteristics which resulted in motor and worm drive drift. Such error was compounded with the disadvantages of conventional auxiliary spring zero adjust mechanisms. It should be appreciated that because the mail processing system of U.S. Pat. No. 3,917,012 dictated high speed, cyclic weighing of successive pieces of mail, instability, i.e. oscillation, of the zero adjust device was undesirable. In order to reduce instability, it was necessary to provide relatively wide dead band and thus sacrifice automatic zero resolution.
Further disadvantages of this prior zero adjust mechanism related to the prior optical readout apparatus itself. The particular transition point wherein the photodetectors switched from conduction to nonconduction states had to be precisely set, and such transition point was highly critical with tolerance requirements rendering final adjustments difficult and time consuming. Furthermore, a relatively large projection path of the detector, e.g. 18 inches-24 inches (45-60 cm.), was required in order to obtain the necessary optical magnification. Such large projection path rendered accuracy of detector readouts and control of the automatic zeroing function highly sensitive to vibrations and other environmental conditions.